The present invention is directed to viscosity meters and particularly to the continuous monitoring type useful in monitoring lubricating fluids.
With the widespread use of pressurized lubricating systems for bearings and the like, need for the continuous accurate monitoring of the condition of the lubricant has long been recognized. A variety of conditions may affect the efficiency of a lubricant which in turn can lead to disastrous consequences if not immediately corrected. For example, lubricant viscosity can be greatly affected by the environment in which the lubricant is used. In many conventional lubricants, the buildup of heat in equipment and specifically in the lubricant itself can cause a substantial lowering of its viscosity. Contamination can also affect the viscosity of the lubricant particularly if a large amount of contaminant is introduced into the lubricating system. Higher viscosities can result from selective vaporization of lubricants while chemical effects can bring about change in either direction. Thus, a wide range of conditions can affect the operative viscosity of a lubricant as it is being used.
The effects of changes in lubricant viscosity on the operation of bearings and the like can be very detrimental to the lubricant's performance and result in expensive and disruptive mechanical failure. This is particularly true with abnormally low lubricant viscosities. With decreased viscosity, the mechanism in which the lubricant is being employed may be unable to supply lubricant fast enough to the bearing surfaces to insure proper lubricating film. A decrease in oil film resonant frequencies can also result in high bearing wear; thus, regardless of the cause of the viscosity change, variations in lubricant viscosity can have a substantial adverse effect on bearing and component life.
Another factor directly effecting any lubricant performance is the pressure at which the lubricant is supplied to the bearing surfaces. As higher pressures are employed, the lubricant is better able to move into otherwise lubricant-starved areas, renew itself promptly enough to remain cool in high load areas and produce higher oil film resonant frequencies in the lubricated mechanisms. Thus, the performance of abnormally low viscosity lubricant can be improved with added pressure.
In the normal use of lubricants, a wide range of viscosity may be experienced. During cold start-up, viscosity may be quite large. During steady state running, the viscosity may approach a range very near a critical point. Consequently, a viscosity monitor may be required which can accurately distinguish between safe and unsafe conditions during steady state running and yet operate under conditions of great disparity in cold starting for example.
A variety of viscosity monitoring devices presently are available. These devices frequently require an input of power, external instrumentation or both. Furthermore, the required instrumentation is often expensive because of the need to provide high resolution as well as a broad range to insure accuracy in determining damaging lubricant conditions.
One common form of viscosity meter employs a capillary tube across which pressure is measured. Examples of such viscosity meter devices are illustrated in Thompson, Jr., et al., U.S. Pat. No. 3,375,704, Uchida et al., U.S. Pat. No. 3,548,638, Detmar et al, U.S. Pat. No. 3,930,402, and Webber, et al., U.S. Pat. No. 4,165,632. The Thompson, Jr., et al. patent also makes reference to another type of viscosity meter device which employs a pressure drop across an orifice as the means for detecting changes in viscosity.